This application claims the benefit of International Application No. PCT/FR00/00871 filed Apr. 13, 1999.
1. Field of the Invention
The invention relates to the in-situ monitoring of the degradation of frying oils and fats (in practice in the food sector).
2. Description of the Related Art
Food oils and fats used in cooking and frying operations undergo a thermo-oxidative alteration leading to a progressive degradation of the oil with the formation of polar compounds.
In France, decree 86-857 of Jul. 18th, 1986 amending the decree of Mar. 11th, 1908 stipulates that fats and oils with a polar compound content greater than 25% are unsuitable for human consumption and provides for the fixing by order of a reference analytical method for determining the polar compound content.
The current standardized reference analytical method is a liquid chromatography at atmospheric pressure followed by a solvent evaporation and a weighing (duration 3 hours). This method cannot be used for routine monitoring.
Moreover, a subjective assessment involving monitoring of the browning of the oil, the viscosity, the appearance of foam or the presence of smoke is not reliable because these criteria can appear well after the 25% limit.
More rapid methods have been developed or envisaged:
A. Methods Based on Colorimetric Reactions
xe2x80x9cLRSMxe2x80x9d test from 3M: this involves soaking a reactive paper in the oil. The criterion defining the end of use of an oil is the number of strips of the reactive paper that have changed color.
xe2x80x9cOXYFRITESTxe2x80x9d test from MERCK-CLEVENOT laboratories: a test tube is filled with oil using a syringe and some drops of reagent are added to it. After a few seconds, the color is modified according to the polar compound content. The limit of use is defined by comparison of the color obtained with a scale of reference colors.
French Patent No. FR 2 513 765 (OIL PROCESS SYSTEMS INC.) for: xe2x80x9cKit and process for the analysis of alkaline substances in fatty foodstuffsxe2x80x9d; the analysis is based on a mixture of fatty material with a titration solution containing a solvent and a colorant.
B. Methods Based on the Measurement of Conductivity or Specific Resistance
U.S. Pat. No. 5,239,258 (KAUFFMAN) for: xe2x80x9cFreshness and Stability Test Using Oxidative Degradationxe2x80x9d; the analysis is based on the development of the current passing through a specimen relative to the voltage applied.
U.S. Pat. No. 5,594,327 (SAGREDOS et al.) for: xe2x80x9cMethod For Determining the Degree of Deterioration of Oils or Fats Used for Frying Foodsxe2x80x9d; the analysis is based on specimens introduced into a measurement cell in which, at a voltage of 100V and a temperature of 75xc2x0 C., the specific ohmic resistance of the material constituting the specimen is measured.
C. Methods Based on the Measurement of the Dielectric Constant
Various scientific articles have already dealt with the problem of degradation of oils, for example:
xe2x80x9cChanges in Dielectric Constant as a Measure of Frying Oil Deteriorationxe2x80x9d, by C. W. FRITSCH et al., published in xe2x80x9cJournal of The American Oil Chemists"" Societyxe2x80x9d, Vol. 56, August 1979,
xe2x80x9cMeasurements of Frying Fat Deterioration. A Brief Reviewxe2x80x9d by C. W. FRITSCH et al. published in JAOCS, March 1981.
These articles showed a strong correlation between the variation in the dielectric constant and the percentage of polar compounds formed. This is exploited in the German Patent Application No. DE 30 06 696 5 (NORTHERN INSTRUMENTS CORP.) providing a device for measuring dielectric properties. The device for measuring dielectric properties which is described therein is sometimes referred to by the name xe2x80x9cFood Oil Sensorxe2x80x9d.
In practice, only the 3M and MERCK-CLEVENOT tests are used. The duration of a test is approximately one minute per deep fryer. The reliability of these tests, however, remains poor because it depends on 3 factors which are difficult to control:
1. storage conditions of the reagents: temperature, time, ambient light;
2. their use:
the reactive papers must not be brought into contact with other objects and must be immersed for a precise time (a few seconds) in the homogenized oil; and
it is vital that the quantity of reagent (a few drops) be observed; and
3. measurement and interpretation:
this must be carried out after a precise period (about ten seconds) and is valid only for a fixed period (about 30 seconds); and
the color changes are progressive and require a degree of experience in order to be read correctly.
As a general rule, all the tests available or mentioned in the literature have the disadvantage of involving sampling, i.e. the taking of samples which are analyzed outside the mass of the fatty material in use. The reliability of the monitoring therefore depends on the periodicity and regularity of the tests.
Of course, the overall reliability also depends on the reliability of the conclusions drawn from the results of the test operations.
All the methods which require prior preparation (chemical, colorimetric, or even electric, U.S. Pat. No. 5,239,258) cannot be used for in-situ monitoring and are hardly suitable for frequent monitoring (typically several times during a heating cycle).
U.S. Pat. No. 5,594,327 describes a method of measuring the variation in conductivity using a DC voltage of 100 V, a method inapplicable for monitoring in a commercial cooking vat (problem of safety voltage).
It must be noted with regard to this latter document that it mentions, among other monitoring techniques, measurement of the dielectric constant of the cooking bath, but considers this method to be complex and costly in terms of monitoring time, without stating a reason. However, the following comments can be made:
A. With Regard to Complexity
The composition of frying oils and fats is always changing, according to clients"" needs and/or the purchase price of the raw materials. For example, an oil can contain animal fats (suet for example), vegetable fats (palm for example), an anti-oxidant (Exxx), and/or an anti-foaming agent (Eyyy). However, the permittivity of such an oil at 180xc2x0 C. (typical usage temperature) does not appear in any publication, including the manufacturer""s technical specifications, nor do its dispersion during successive production stages, or its variation when it contains 25% of polar compounds.
Moreover, each catering group uses an oil or a fat the composition of which it has determined itself.
B. With Regard to the Cost in Terms of Monitoring Time
The above-mentioned patent doubtless refers to the Food Oil Sensor which proceeds by sampling and that has clearly proven to be long and complex.
An object of the present invention is to overcome the disadvantages of the known solutions, providing a process for monitoring that is at once reliable and repetitive, and that lends itself to automation so as to be able to guarantee in particular the consequences of the detection of too large a quantity of polar compounds.
To this end, the present invention proposes a process for monitoring a frying vat containing a bath of frying oil or fat intended to follow a plurality of heating cycles, according to which:
A. In a zone of the vat suitable for immersion in the bath, a capacitive probe is arranged, the dielectric space of which is constituted by a portion of the bath, and, after filling the vat with this bath, an initialization procedure is triggered during which an initial value of a characteristic representative of the dielectric constant of this bath in heating conditions is measured, and a shutdown criterion is fixed.
B. A measurement of this characteristic is repeatedly detected during the following heating cycles, and a shutdown procedure is triggered when it is detected that the shutdown criterion is met by this measurement.
The invention thus teaches the use of a capacitive, in practice permanent, probe in the cooking vat. The probe must therefore withstand an average temperature of 180xc2x0 C. and its construction must satisfy the food and electric standards. The requirements for the probe can be expressed as follows:
high temperatures: 180xc2x0 C. with a maximum of 210xc2x0 C.;
suitability for food preparation; and
sturdiness: daily cleanings and filterings of the vat by non-qualified personnel.
However, it appears that a capacitive probe is completely suitable for such conditions, taking into account its simplicity (two electrodes separated by a dielectric space immersed in the bath). Moreover, the act of comparison with the initial value of the characteristic in question makes the process reliable irrespective of a certain number of parameters that are difficult to control, and vary from one oil bath to another.
In large-capacity commercial deep fryers, the sensor does not need to be miniaturized. There is a dead zone in the cooking vat that can be used to fit a capacitive sensor of sufficient dimensions to guarantee its sturdiness and to characterize a substantial part of the oil bath that is representative of same.
Therefore, contrary to what might have been supposed beforehand, there are no particular difficulties with the permanent (in situ) installation of a capacitive probe in a frying vat.
The advantages of the proposed solution, compared with the devices known under the name xe2x80x9cFood Oil Sensorxe2x80x9d are therefore:
repetitive in-situ -monitoring of the oil in the cooking vat; and
possibility of automatic monitoring without human involvement (this process is in fact easily automatable by means of closed-loop controls).
The process according to the invention can use various particularly advantageous arrangements, taken separately or in combination.
The detection of the measurements is advantageously periodic, if the instantaneous value of the characteristic is measured (or if an average of such instantaneous values is taken) at least once an hour (this is an order of magnitude corresponding to several measurements per heating cycle), it can be stated without difficulty that the monitoring is continuous, taking into account the speed of degradation of the oil baths (the length of their operational life is in fact typically more than 60 hours).
A simple procedure is to measure the characteristic by exciting an oscillating circuit of which the capacitive probe is part.
Various characteristics of the oscillating circuit can be used. In simple manner, the oscillation period is measured, i.e. the characteristic is measured by excitation of an oscillating circuit of which the capacitive probe is part, and said characteristic that is measured is the oscillation period of the oscillating circuit.
The measurement that is detected can be the average of several successive measurements which has the advantage of producing a smoothing effect.
The shutdown criterion preferably involves a comparison with the initial value of the characteristic.
The comparison of the measurement with the initial value of the characteristic that has been chosen can be carried out by establishing their ratio. However, in a particularly simple manner, the shutdown criterion is the exceeding of a set value threshold by the difference between the xe2x80x9cinstantaneousxe2x80x9d measurement and the initial value of the characteristic. By way of a variant, this shutdown criterion can also be defined starting from an absolute variation of this measurement from the characteristic.
To allow the user to take steps in advance to replace a bath the life of which is nearly over, an alert criterion is also advantageously fixed (the latter may or may not be defined in relation to the initial value of the characteristic), and an alert procedure is triggered (it can be sound or vision only, without influencing the operation of the vat) when it is detected that this alert criterion is met.
In a preferred manner, when the shutdown criterion is fixed by the difference between the initial and instantaneous values, the alert criterion is advantageously fixed, in relation to the initial value of the characteristic, by a difference threshold which is less than the difference defining the shutdown criterion, and an alert procedure is triggered when this intermediate threshold is exceeded.
To minimize the possibly sudden character of the implementation of the process according to the invention, above all when the latter is automated, the shutdown procedure is advantageously inhibited until the heating cycle following that during which the shutdown criterion has been detected. There is therefore no difficulty in terminating the heating cycle in progress as normal, which avoids any untimely interruption to the supply of the fried products in the vat.
The shutdown procedure can include stoppage of the heating cycle following that during which the shutdown criterion has been detected, after a predetermined time (possibly zero) intended to allow the detection of the completion, or lack thereof, of bath replacement steps and the acquisition of the data required for the monitoring of this new bath.
It is easily understood that the reliability of the monitoring depends on the elimination of possible causes of ill-timed shutdowns. This is why, advantageously, account is taken only of measurements of the characteristic that are carried out when the temperature of the bath is at least equal to the threshold; it is thus that, in a preferred manner, a characteristic representative of the temperature of the bath is also detected, and the measurements of the characteristic or the monitoring of the shutdown criterion are inhibited when the temperature of the bath is lower than a minimum temperature threshold (if the measurement involves taking the average of several data acquisitions, each of these data acquisitions must be carried out at a temperature higher than the threshold).
To be able to benefit from past experience, and to be able to ensure a monitoring of the vat over a long period, including during the changing of the bath, data relating to previous baths is preferably stored.
When it is detected that, after a predetermined time after the start of a heating cycle following a heating cycle during which the shutdown criterion has been established, there has been no initialization procedure, the interruption of the heating cycle is provoked. This actually reflects a possible error in the behaviour of the user.
Any initialization procedure triggered after a predetermined period after the start of a heating cycle following a heating cycle during which the shutdown criterion has been established is inhibited. This prevents any attempt to unduly prolong the duration of the life of a bath by taking a new initialization value without changing the bath.
The initialization procedure is defined in relation to at least one of the stored initialization procedures of the previous baths, which makes use of the comments concerning the behavior of the previous baths so that they can, for example, be defined by precise measurement of the polar compound content.
The initialization procedure can advantageously be triggered only from a box independent of the vat. It is therefore possible to entrust this box only to an authorized person, and to thus clearly fix the responsibilities regarding the replacement of baths that have reached the end of their lives.
The invention also teaches a device for monitoring a frying vat containing a bath of frying oil or fat intended to follow a plurality of heating cycles such that the device is suited to the implementation of the process. The device for monitoring includes:
a capacitive probe arranged in a zone of the vat suitable for immersion in the bath;
a measurement and processing unit connected to the probe and to the electricity supply of the vat, and suitable for the measurement of values of a characteristic representative of the dielectric constant of this bath in heating conditions, and conceived so as to:
store, in response to an initialization command, an initial value of this characteristic and a shutdown criterion; and
detect, repeatedly during the heating cycles of the bath, a measurement of this characteristic, and to trigger a shutdown procedure when it is detected that the shutdown criterion is met by this measurement of the characteristic, and
an initialization unit suitable for connection to the measurement and processing unit for transmitting an initialization command and initialization data concerning the shutdown criterion to the measurement and processing unit.
By analogy with the preceding comments concerning the process, according to preferred teachings of the invention, optionally combined:
the measurement and processing unit is conceived so as to measure the characteristic at least once an hour (i.e. several times per heating cycle);
the probe is fitted in an oscillating circuit, and the measurement and processing unit is conceived so as to measure the instantaneous value of a characteristic of this oscillating circuit;
the characteristic is the oscillation period of the oscillating circuit;
the measurement which is detected is an average of several successive measurements;
the measurement and processing unit is conceived so as to be able to detect the exceeding of a set value threshold defined by the initialization unit within the data concerning the shutdown criterion, by the difference between the measurement and the initial value of the characteristic;
the measurement and processing unit is also conceived so as to store an alert criterion (for example in relation to the initial value of the characteristic), defined by the initialization unit, and to trigger an alert procedure when it is detected that this alert criterion is met by the measurement of the characteristic;
the measurement and processing unit is conceived so as to inhibit the shutdown procedure until the heating cycle following that during which the shutdown criterion has been detected;
the shutdown procedure includes a method for stoppage, by cutting the supply to the vat, of the heating cycle which follows that during which the shutdown criterion has been detected, after a predetermined time;
this device also has a detecting element for measuring a characteristic representative of the temperature of the bath, and the measurement and processing unit is connected to this detecting element and is conceived so as to inhibit the measurements of the characteristic or the monitoring of the shutdown criterion when the temperature of the bath is lower than a minimum temperature threshold;
the measurement and processing unit is also conceived so as to store data relating to previous baths;
the measurement and processing unit is conceived so as to be able to detect that, after a predetermined time after the start of a heating cycle following a heating cycle during which the shutdown criterion has been established, there has been no transmission of an initialization command or data from the initialization unit, and to thus be able to trigger the interruption of the heating cycle by cutting the supply to the vat;
the measurement and processing unit is also conceived so as to inhibit any initialization command or data from the initialization unit, after a predetermined period after the start of a heating cycle following a heating cycle during which the shutdown criterion has been established;
the initialization unit is conceived so as to define the initialization data in relation to the initialization data and data exchanged with the measurement and processing unit for at least one of the previous baths; and
the initialization unit is detachably connected to the measurement and processing unit.
In addition, to clearly separate the measurement and processing functions, and command functions (via the initialization command and the choice of initialization data), the measurement and processing unit is preferably fixed to the vat and the initialization unit is contained in a detachable box.
In the case of an electric deep fryer, the capacitive probe is preferably fixed in the vat below resistors with which the vat is fitted for the heating of the bath. This is in fact a site available for fitting the probe, one that in addition allows an easy fixing of the latter, and an easy connection to the outside.
In a particularly simple manner, the capacitive probe has two electrodes one of which is at the constant potential of the vat.